Class Builds (Diablo II)
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A class build is a generalized way of describing the way a character will fight. Accustomed players will be able to deduce not only what skills a build uses when seeing or hearing the name of a build, but will also know how a character will fight in game and roughly what character stats and gear will look like of that build. For example, the following will make the term Hammerdin obvious: It's a Paladin class focusing on the spell Blessed Hammer.
New and old players alike often refer to friends or internet for class builds when it comes to creating a new character. The way a character is developed is easily formatted into writing, giving insight to other players on how and why a build should be built like it is. The way a Class Build Guide is formatted varies depending on the author. Some guides will go as far as recommending certain Jewels to be socketed, while other guides will list the skill setup and give a few short pointers on where stats should be going, leaving more room for preferred gear setup. In the end, a player who starts understanding how D2 game mechanics work will be able to read a guide and tweak it for his own use, having a rough but helping guideline from the beginning of a character's development.
The term "Funbuild" means that a character is able to be played a specific way that is more uncommon, but usually in an interesting manner. These builds are not always perfect for general PvE, but can be played successfully with skill. A melee Sorceress using Enchant is still about killing, but doesn't perform as well as a straight caster in some cases. However, this build can still provide many hours of gameplay. Other builds like the Suicide Necro, are even less practical in terms of playing, but can lead to many amazed looks from other players when the Necromancer lets himself be killed, activating multiple Death Rainbow Facets at once.
 Theory Crafting
Expert players trying out new things or attempting to perfect a character tend to theory craft instead of attempting something without any deep thoughts. In simple terms, theory crafting in this context means planning out possible cause and effect attributes of a character. A well developed knowledge of the inner game workings is required, such as knowing how gear affects damage and survivability, solutions to possible immunity problems, stat requirements, some simple RNG based values, and thorough understanding of what certain attributes found on gear and skills can actually do to improve your character. A lot of time and mathematical thinking can go into theory crafting, resulting in a generally well thought out thesis of a character's performance in the game. With some effort, the actual characters resulting out of this planning can wipe the enemies out easily compared to more simplistic builds, or even of the same type of build.
A common form of theory crafting happens when a player looks at a guide and thinks of available assets compared to recommended assets, such as gear to follow through with character development. Through this, a player can work towards the best character setup using things that are available to him or her. This can mean a character with less than optimal gear than recommended can still be as effective, or even better than suggested if skills are pushed towards the play style instead of strictly following the guide.
Another form of theory crafting is developing a character with other gear in mind than currently available, while still making the character playable at the moment without spending more stats than the build will ultimately need at the end.